The present invention relates to digital communication systems and more particularly to optimizing operation where received signal power varies rapidly.
In a wireless environment, the received signal energy can vary over many orders of magnitude due to variations in the characteristics of the communication channel. Moreover, the variations in received signal strength are typically frequency selective. Digital communication systems have until recently principally relied on single carrier quadrature amplitude modulation (SC-QAM). Such systems employ an analog automatic gain control (AGC) loop and an adaptive equalizer to handle the variations in received signal strength. The mean signal power is controlled by the AGC loop while the digital adaptive equalizer attempts to handle the frequency selective gain variations. Such equalizers vary their frequency response based on ongoing measurements of channel characteristics. However, even such systems can accommodate rapid channel variations only by employing a low-density constellation thus greatly limiting the number of data bits communicated by each symbol.
A new type of digital communication system based on orthogonal frequency division multiplexing (OFDM) copes far more effectively with rapid channel variation. OFDM divides the available spectrum within a channel into narrow subchannels. In a given so-called xe2x80x9cburst,xe2x80x9d each subchannel transmits one data symbol. Each subchannel therefore operates at a very low data rate compared to the channel as a whole. To achieve transmission in orthogonal subchannels, a burst of frequency domain symbols are converted to the time domain by an IFFT procedure. To assure that orthogonality is maintained in dispersive channels, a cyclic prefix is added to the resulting time domain sequence. The cyclic prefix is a duplicate of the last portion of the time domain sequence and is appended to the beginning.
To handle varying channel characteristics, some of the frequency domain subchannels carry training symbols that have fixed values known at the receiver. The receiver can estimate and correct for the channel response at each burst by measuring the received values of the training symbols. This process can handle rapid channel variation far more effectively than the adaptive equalizers employed by SC-QAM systems.
However, correction of symbol values for measured variation in channel response is only one aspect of handling rapid time variation. The IFFT procedure, channel estimation, and correction for measured channel response are all typically accomplished by digital signal processing. Additionally, there is usually also digital filtering. A digital to analog converter precedes the digital stages in the receiver chain. The analog signal at the converter input must be kept within a range that assures accurate converter operation.
Prior art digital communication receivers incorporate automatic gain control loops that vary the mean analog receiver gain to assure proper operation of the digital to analog converter. Though the adjustment speeds of these loops can be very high, the performance of the SC-QAM system is limited by the capability of the digital adaptive equalizer in handling time varying signals, i.e., frequency selective signal variations. Improvements in receiver automatic gain control are required to take advantage of the superior performance of OFDM in time varying channel environments.
Further challenges arise when considering the application of OFDM to point to multipoint communication systems that interconnects a central access point with multiple subscriber units. Multiple OFDM signals may be frequency multiplexed onto adjacent channels. To minimize complexity and cost, the analog receiver stages have a bandwidth that includes all of the multiple signals. These signals will then be separated from one another by digital filters between the analog to digital converter and parallel IFFT stages that operate independently on each of the multiple signals. For correct operation and to minimize the needed arithmetic precision, input to each of the IFFT stages should be within a defined range. An automatic gain control loop that sets gain based on the analog to digital converter input level only will be unable to also assure optimal signal processing operation.
Improved receiver automatic gain control for OFDM-based communication systems is provided by virtue of one embodiment of the present invention, thus realizing the full potential of OFDM systems to handle rapid variation in channel characteristics. The improved automatic gain control technique can also accommodate reception of multiple OFDM signals having disparate power levels via the same analog receiver chain and analog to digital converter while minimizing the arithmetic precision necessary for optimal digital processing. By reducing the necessary arithmetic precision, the cost of implementing the receiver with integrated circuit technology is greatly reduced.
According to one aspect of the present invention, a digital communication receiver includes: an analog receiver system that receives an analog signal and amplifies the analog signal responsive to a first gain control level, an analog to digital converter that converts the amplified analog signal to a digital signal, a digital filter that filters the digital signal to select the desired digital information signal, a scaling element that digitally scales the desired digital information signal according to a second gain control level, a transform element that transforms the desired digital information signal into the frequency domain, and a first gain control loop that adjusts the first gain control level to optimize operation of the analog to digital converter.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portion of the specification and the attached drawings.